Entrained gas phase content in liquids can be measured by various direct and indirect methods. Some indirect methods include density, viscosity, and attenuation of sound waves. In general, indirect methods suffer limitations due to contributions by other factors. For example, density also depends upon general composition, and attenuation of sound waves depends upon the presence of suspended solids. Direct measurement can rely upon fluid compressibility. Simply stated, liquids are incompressible while gases are compressible. Some of the prior methods used for laboratory measurements of entrained air in process streams utilize direct measurement by expansibility (inverse compressibility) of isolated process stream samples subjected to reduced pressure. See, for example: J. D. Boadway, Gas in papermaking stock, Pulp and Paper Magazine of Canada, Convention Issue, 57 (3): 185-189, 194 (1956); E. Parker Troland, Measuring suspended air in paper stock, Tappi, 49 (9): 100A-102A (1966); P. Landmark, "New apparatus for determining entrained air in pulp suspension," Norsk Skogind 21 (12): 503-506 (1967); E. Barkowski, "Air in aqueous stock suspensions-methods for determining air content," Papiermacher 28, (3): 44-46 (1978).
All of these methods provide a direct measurement of the entrained air content by means of collection and isolation of a sample, subjecting the sample to reduced pressure, measurement of the expansion of the sample due to the expansion of the entrained gas phase, and, by means of common thermodynamic equations of state, calculation of the fraction of sample which is entrained gas phase.
None of the methods described above are capable of providing on-line automatic process measurements due to a number of factors, including for example, fragile construction, plugging with particulate material, and leakage of sealing valves.
The present inventor in his earlier U.S. Pat. No. 4,700,561 describes an apparatus for the direct measurement of entrained gasses by means of gas phase compressibility. The apparatus is suitable for on-line measurement in industrial environments. Other devices for the direct measurement of entrained gas phase by means of fluid compressibility have been described in the following U.S. Pat. Nos. Cromer et al., 2,138,141; Jones, 3,911,256; and Toda, 4,329,869. Without exception, all of these procedures require the collection and isolation of a liquid process sample.
Holzl, U.S. Pat. No. 4,365,505, describes an apparatus which uses a variation of the compressibility principle, while not requiring the collection and isolation of a liquid process sample. Holzl's apparatus relies upon repetitive pressure pulses induced by various means to induce harmonic pressure pulses in liquids containing compressible gases. The effect arises as a consequence of a double-mass oscillator effect wherein the compressible gas is serving as the spring. The concept depends also upon the mass of the fluid, and it is therefore susceptible to other factors that influence the coupling of the primary pressure pulses to the fluid and the subsequent propagation of the pulses through the fluid. These factors can include viscosity, suspended solids, and piping geometry.
The present invention describes a concept and an apparatus for on line determination of the entrained gas-phase content of liquids by means of the direct compression of process fluid, independent of the process geometry and fluid composition, and without the need to obtain process samples.